1. Field of the Invention
The present invention relates to an apparatus for use in a video camera including a lens system composed of a plurality of lens groups and, in particular, to an automatic focus control apparatus for a video camera which can correct a defocus as produced in a lens system.
2. Description of the Related Art
A conventional automatic focus control apparatus for use in a video camera achieves a just-in-focus state as will be set out below. That is, a defocus detection circuit detects a defocus signal corresponding to a defocus from an image signal obtained at an image pick-up element via a lens system. The defocus signal is fed back to a motor to move part of the lens system. Here, the lens system is of such a type that, for example, a focus lens group, a zoom lens group, a focal position correction lens and a relay lens group (or a coupling type lens group) are arranged in that order as viewed from a subject to be imaged. Normally, the focus lens group is moved by a motor in the aforementioned arrangement to gain a "just-in-focus" state. Of these four lens groups, the focus lens group is the largest. Further, an amount of correction by which a defocus resulting from the movement of the focus lens is corrected is small. It is, therefore, necessary to move the larger lens group to a greater extent so that a just-in-focus state may be attained. Moving the focus lens group to achieve the in-focus state in this way requires a complex structure for associated parts or mechanisms of the optical system. In addition, the focus lens group also involves a greater amount of movement, making it difficult to provide a compact, light-weight video camera.
Recently, an automatic focus control apparatus for a video camera has been proposed to obtain a just-in-focus state by moving an image pick-up element in place of the lens group. Stated in more detail, the image pick-up element is of such a type that it is movable by a piezoelectric actuator, that is, it is moved by feeding a defocus signal back to the piezoelectric actuator-Published Unexamined Japanese Patent Application 61-54788. Since, in the aforementioned apparatus, the image pick-up element is moved by the piezoelectric actuator, it is not necessary to provide, for example, a motor and a mechanism for converting a rotational motion of the motor to a linear motion of a focus lens group. This feature simplifies the associated mechanism of the lens system and ensures a compact, light-weight, and easily-operable video camera.
In actual practice, a stroke at which the image pick-up element is moved to obtain an in-focus state is normally as great as an order of a few mm. In the piezoelectric actuator, such as a bimorph element, it is very difficult to achieve a stroke of the order of a few mm. Thus the apparatus as set out immediately above involves a greater burden which is borne by the piezoelectric actuator.
In the apparatus for moving the focus lens group by the motor to gain an in-focus state, the associated parts or mechanisms become complicated in structure and the focus lens group is moved at a greater stroke, failing to implement a compact, light-weight video camera. In the apparatus for moving the image pick-up element by the piezoelectric actuator, such as a bimorph element, to give an in-focus state, a greater stroke burden is borne by the piezoelectric actuator.